GRIN2B mutations in West syndrome and intellectual disability with focal epilepsy.
To identify novel epilepsy genes using a panel approach and describe the functional consequences of mutations. Using a panel approach, we screened 357 patients comprising a vast spectrum of epileptic disorders for defects in genes known to contribute to epilepsy and/or intellectual disability (ID). After detection of mutations in a novel epilepsy gene, we investigated functional effects in Xenopus laevis oocytes and screened a follow-up cohort. We revealed de novo mutations in GRIN2B encoding the NR2B subunit of the N-methyl-D-aspartate (NMDA) receptor in 2 individuals with West syndrome and severe developmental delay as well as 1 individual with ID and focal epilepsy. The patient with ID and focal epilepsy had a missense mutation in the extracellular glutamate-binding domain (p.Arg540His), whereas both West syndrome patients carried missense mutations within the NR2B ion channel-forming re-entrant loop (p.Asn615Ile, p.Val618Gly). Subsequent screening of 47 patients with unexplained infantile spasms did not reveal additional de novo mutations, but detected a carrier of a novel inherited GRIN2B splice site variant in close proximity (c.2011-5_2011-4delTC). Mutations p.Asn615Ile and p.Val618Gly cause a significantly reduced Mg(2+) block and higher Ca(2+) permeability, leading to a dramatically increased Ca(2+) influx, whereas p.Arg540His caused less severe disturbance of channel function, corresponding to the milder patient phenotype. We identified GRIN2B gain-of-function mutations as a cause of West syndrome with severe developmental delay as well as of ID with childhood onset focal epilepsy. Severely disturbed channel function corresponded to severe clinical phenotypes, underlining the important role of facilitated NMDA receptor signaling in epileptogenesis.
PubMed ID: 24272827
PMC ID: PMC4223934
Article link: Ann Neurol.
Grant support: MR/J004049/1 Medical Research Council
Genes referenced: bcl9 grin1 grin2b nodal2
Article Images: [+] show captions
|Figure 1. Location of GRIN2B mutations in a schematic illustration of the conserved domains of the NR2B subunit (SP = signal peptide; ATD = amino-terminal domain, involved in receptor assembly; S1 and S2 form the ligand-binding domain; Pore = re-entrant pore-forming and transmembrane spanning domains; PDZ = PDZ domain binding motif). All reported de novo mutations and their according phenotypes (ASD = autism spectrum disorders; FE = focal epilepsy; ID = intellectual disability; LGS = Lennox–Gastaut syndrome; Scz = schizophrenia) are listed in the top row. Mutations causing phenotypes without seizures are labeled in black, mutations in epilepsy patients are in red. So far, no pathogenic variants have been observed in the C-terminal region of NR2B. Mutations causing West syndrome cluster within re-entrant pore-forming domain, whereas the mutation causing ID and focal epilepsy was observed in the glutamate-binding domain S1, similar to a recently described LGS case. Nonsynonymous variants that are believed not to be associated with abnormal phenotypes (gray) and are reported more than once (in brackets) in the Exome Variant Server (EVS) are listed in the bottom line.|
|Figure 3. Structural and functional analyses of the glutamate binding-domain mutation Arg540His. (A) Residue 540 is predicted to be located within the glutamate-binding S1 domain, and is significant in the stabilization of the tertiary structure of the glutamate-binding domain. (B) Substitution p.Arg540His is likely to abolish hydrogen bonding (blue lines) with the backbone of Cys746 and His802 and a cation–pi interaction with His802, possibly leading to a relaxed fold in this region. (C) Pharmacological characterization of the apparent agonist affinities of wild-type NR1-NR2B (black triangles) and mutant NR1-NR2BArg540His (red squares) N-methyl-D-aspartate receptors measured after heterologous expression in Xenopus laevis oocytes by 2-electrode voltage-clamping revealed that similar glutamate concentrations were required to elicit half-maximal responses (EC50 values = 0.72 ± 0.22μM and 0.31 ± 0.02μM, respectively, p = 0.14, n = 3). (D) Arg540 is a highly conserved residue within NR2A–D subunits.|